This invention relates generally to methods and apparatus used to produce precision optical elements, and more particularly to such methods and apparatus for molding optical elements having complex and concave surfaces.
Precision optical elements require highly polished surfaces of exacting figure and surface quality. The surfaces demand fabrication in proper geometric relation to each other and, where the elements are to be used in transmission applications, they will be prepared from a material of controlled, uniform, and isotropic refractive index.
Precision optical elements of glass are customarily produced via one of two complex, multi-step processes. In the first, a glass batch is melted in a conventional manner and the melt formed into a glass body having a controlled and homogeneous refractive index. Thereafter, the body may be reformed utilizing well-known repressing techniques to yield a shape approximately the desired final article. The surface figure and finish of the body at this stage of production, however, are not adequate for image forming optics. The rough article is fine annealed to develop the proper refractive index and the surface figure improved via conventional grinding practices. In the second method, the glass melt is formed into a bulk body which is immediately fine annealed and substantially cut and ground to articles of a desired configuration.
Both processes are subject to similar limitations. The surface profiles that are produced through grinding are normally restricted to conic sections, such as flats, spheres, and parabolas. Other shapes and, in particular, general aspheric surfaces are difficult to grind. In both processes, the ground optical surfaces are polished employing conventional, but complicated, polishing techniques which strive to improve surface finish without compromising the surface figure. In the case of aspheric surfaces, this polishing demands highly skilled and expensive handworking. A final finishing operation, viz, edging, is commonly required. Edging insures that the optical and mechanical axes of a spherical lens coincide. Edging, however, does not improve the relationship of misaligned aspheric surfaces, which factor accounts in part for the difficulty experienced in grinding such lenses.
The direct molding of lenses to the finished state could, in principle, eliminate the grinding, polishing, and edging operations which are especially difficult and time consuming for aspheric lenses. Indeed, molding processes are utilized for fabricating plastic lenses. Existing plastics suitable for optical applications are, nevertheless, only available in a limited refractive index and dispersion range. Furthermore, many plastics scratch easily and are prone to the development of yellowing, haze, and birefringence. The use of abrasion-resistant and antireflective coatings has not fully solved those failings. Moreover, plastic optical elements are subject to distortion from mechanical forces, humidity, and heat. Both the volume and refractive index of plastics varies substantially with changes in temperature, thereby limiting the temperature interval over which they are useful.
The overall properties of glass render it generally superior to plastic as an optical material. Glass is a much better substrate for the application of multilayer anti-reflection coatings because it is chemically inert, dimensionally stable, and can be coated at elevated temperatures. As described above, glass also has excellent environmental performance over a broad range of temperature, humidity, and other environmental conditions. This performance is due to its low coefficient of thermal expansion, its essential imperviousness to water absorption, its high resistance to other environmental attacks (e.g., salt spray, fungus, and acids), and its resistance to other atmospheric contaminants. Additionally, glass has a very high mechanical strength, allowing precision optical elements formed of glass to perform without optical degradation, or mechancial deformation while under stress. Conventional hot pressing of glass, however, does not provide the exacting surface figures and surface qualities demanded for image forming optics. The presence of chill wrinkles in the surface and surface figure deviations constitute chronic problems. As observed above, similar problems can be encountered in conventional repressing techniques.
Various processes have been devised to correct those problems, such processes frequently involving isothermal pressing, i.e., utilizing heated molds so that the temperature of the glass being molded will be essentially the same of that of the molds, the use of gaseous environments inert to the glass and mold materials during the pressing operation, and/or the use of materials of specifically defined compositions in the construction of the molds.
U.S. Pat. No. 4,481,023--Marechal and Maschmeyer shows and describes an improved mold for precisely pressing a glass preform which has an overall geometry similar to the desired final lens. A top and a bottom mold have molding cavities which precisely match the configuration of the final lens. A glass preform is heated to the molding temperature and the mold parts are separately heated. The molds are brought together against a ring having a thickness which governs the thickness of the lens to be molded.
In such a molding operation, the molding of the two opposed optical surfaces should be balanced. Balanced molding of a lens means that the degree to which the glass fills the voids between both top and bottom mold surfaces is equivalent. This is typically measured by the radii between each lens surface and the common sidewall. Copending U.S. patent applications Ser. Nos. 940,120, filed Dec. 10, 1986, entitled "Kinematically Determinate Mold Assembly", and 071,405 filed July 9, 1987, entitled "Balanced Molding of Optical Elements", respectively show and claim a mold assembly which can be controlled in accordance with a prescribed balanced molding technique utilizing adjustments to the closing movement of the molds.
The molding of aspheric lenses in accordance with the above described references requires that a precise preform, or blank, be produced with two polished surfaces. These precise preforms are then pressed in a mold to the final finished form. For example, for small lenses for audio and video players, the preform shape is a bi-convex lens of about 7 to 14 millimeters in diameter. Computer modeling of glass flow is generally used to assist in the development of the precise preform shape. Thereafter, one of two processes are typically used to produce the preform. The first involves a grinding process to produce the general shape of the preform, followed by conventional lapping and polishing steps to produce the required finish on the lens preform. In the second process, as shown and claimed in copending U.S. patent application Ser. No. 940,275, filed Dec. 10, 1986, entitled "Simultaneously Grinding and Polishing Preforms for Optical Lenses", a ground and polished optical surface on a glass lens preform is produced in a single operation. Either method, however, is limited in its ability to prevent unfinished surfaces or flashing in complex and concave mold surfaces without the use of preforms of a precise shape. In fact, there are shapes for which no simple preform can be made that will yield finished surfaces without flashing. Such considerations become critical in the production of precision optical elements as shown and claimed in copending U.S. patent application Ser. No. 075,573, filed July 20, 1987, entitled "Expanded Beam Waveguide Connector".
It is, therefore, a general object of the present invention to provide a method and apparatus for molding precision optical elements by completing the molding operation of their two optical surfaces at substantially the same time. More specifically, it is an object of the present invention to provide a method and apparatus for molding precision optical elements without the necessity of utilizing precise and complicated preforms.
It is another object of the present invention to provide a method and apparatus of molding precision optical elements having complex and concave optical surfaces without the formation of unfinished surfaces and flashing.
These and other objects of the present invention are provided by independently controlling the motion of pressing tools in an alignment sleeve. In accordance with the present invention, opposing optical surfaces are formed substantially at the same time from an essentially arbitrarily-shaped preform by apportioning the total relative motion of their respective molds towards one another as some motion of one mold within the alignment sleeve and some motion of the other mold within the alignment sleeve. Such apportioning may be controlled by apparatus according to the present invention in which a calibrated standoff is attached to the alignment sleeve in order to limit the movement of one mold after its respective molding surface has contacted the arbitrarily-shaped preform.
The foregoing and other objects, features and advantages of the present invention will be better understood from the following more detailed description, when considered in conjunction with the accompanying drawings wherein: